Windows: National Geographic's web site has some stunning photography, and a lot of it makes for good rotating desktop backgrounds. The NatGeo Wallpaper Downloader we've previously enjoyed has updated with interface improvments, bug fixes, and access to thousands more images.
The app's developer writes that while his first release of the app made it convenient to grab a few hundred of National Geographic's photography contest images, this 2.0 version broadens your access to more than 5,000 images. Future versions should expand the range even further. It's a Windows-only app, but Linux and Mac users with some command line savvy can still get in on the NatGeo action.
NatGeo Wallpaper Downloader is a free download for Window systems.